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Pierre de Castelnau (? - died 15 January 1208), French ecclesiastic, was born in the diocese of Montpellier.
He was archdeacon of Maguelonne, and in 1199 was appointed by Pope Innocent III as one of the legates for the suppression of the Cathar heresy in Languedoc. In 1202, he became a Cistercian monk at the abbey of Fontfroide, Narbonne, and was confirmed as Apostolic legate and first inquisitor, first in Toulouse, and afterwards at Viviers and Montpellier.
In 1207 he was in the Rhone valley and in Provence, where he became involved in the strife between the count of Baux and Raymond VI, Count of Toulouse. Castelnau was assassinated on 14 January 1208, quite possibly by an agent of Raymond. His murder was the immediate cause of Raymond's excommunication and the start of the Albigensian Crusade.
He was beatified in the year of his death by Pope Innocent III, who held Raymond responsible. The relics of Pierre de Castelnau are interred in the church of the ancient Abbey of St-Gilles.